Light sources having high light flux and high luminance are employed in a wide variety of fields, for instance in endoscopy as well as in projection appliances, for which purpose gas discharge lamps are currently most widespread. In the case of illumination applications, for example projection or endoscopy, based on LARP (“Laser Activated Remote Phosphor”) technology which is known in principle, a phosphor is excited by a laser which is arranged remotely from the latter. In this case, the laser radiation which strikes the phosphor is at least partially converted by the phosphor by means of wavelength conversion into wavelength-converted useful light.
Rotating phosphor wheels are typically used as phosphors in order to distribute the laser beam output of the incident laser radiation averaged over time over a larger surface area and thus to reduce the phosphor degradation. A phosphor wheel has at least one sector that is coated with a phosphor. The sector may be a circular sector or a circular ring sector. A plurality of sectors may be arranged sequentially, radially and/or in circumferential direction on one side of the phosphor wheel. The sectors are typically coated with different phosphors which convert the incident laser radiation, for example ultraviolet (UV) radiation or blue-violet light, into radiation in another wavelength range, for example to light in the red (red phosphor), yellow (yellow phosphor), green (green phosphor) or blue (blue phosphor) wavelength range. The temporally sequential colored light channels for supplying an imaging unit, which are necessary for video projection, can also be produced using the successive color phosphor sectors of the rotating color wheel.
The light which is wavelength-converted by the phosphors is typically collected using an optical apparatus, for example reflector, converging lens or TIR optics (TIR: Total Internal Reflection; for example conical glass rod) and used further for the particular application.
One disadvantage is that red phosphors, in contrast to yellow and green phosphors, have a lower conversion efficiency when they are irradiated with laser radiation with high surface power densities (for example 10-50 W/mm2) As a result, in particular for red light, the light flux and luminance which can be achieved using LARP technology are limited. In addition, a few efficient phosphors have dominance wavelengths, which are not optimum for some applications, in particular video projection. The dominance wavelength (also referred to as dominant wavelength) is defined by light of a light color (colored light) in the CIE color diagram (standard color table) by the point of intersection of the straight line which is extended from the white point over the ascertained color locus of the colored light with the spectral locus at the closest edge of the CIE color diagram. Efficient red phosphors, for example, have only a dominance wavelength of about 600 nm. However, for projection uses, red light with a dominant wavelength in the range of approximately 600 to 620 nm is typically needed.